This invention relates to optical systems having a dome window and an optical system with an imaging lens such as a solid catadioptric imaging lens, and, more particularly, to avoiding ghost images that may otherwise be visible at the sensor of such optical systems.
An optical system includes an optical train with a sensor that receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for automated pattern recognition or the like. The sensor is fragile and is easily damaged by dirt, erosion, chemicals, or high air velocity.
In service, the sensor is placed behind a transparent, dome-shaped window through which it views the scene and which protects the sensor from such external effects. In some cases, the dome may be spherical. If the dome is nonspherical, highly curved, and thick, it introduces significant wavefront aberration into the optical rays that pass through it on the way to the sensor. As discussed in U.S. Pat. No. 6,028,712, a transparent optical corrector in the optical path may be placed between the dome and the sensor to compensate for the aberration introduced by the nonspherical window.
In a number of such optical systems, a catadioptric (also sometimes termed xe2x80x9ccatadiopticxe2x80x9d) imaging lens is used in the optical train. Light passing through the dome and the optical corrector, if any, is refracted and reflected by the catadioptric lens to the sensor. The catadioptric lens combines a transmissive entrance window, a reflective primary, a reflective secondary, and a transmissive exit pupil into a single optical element.
Although the catadioptric lens is quite efficient, it and other types of lenses suffer from the problem that reflections from the dome or the optical correctors, if any, and from the front side of the catadioptric lens itself may reach the sensor as a ghost image. An analogy, although somewhat imperfect, is the pattern that may sometimes be seen as the reflection from the windshield by the driver of an automobile. Under the right light conditions, the driver may see reflections in the windshield of the interior of the automobile or of objects outside the automobile that are not in the viewed scene. The pattern recognition system of the human mind can normally distinguish the viewed scene from the reflected pattern, but the pattern recognition systems of presently available image processors are not that sophisticated. This type of stray-light image is thus extremely difficult to distinguish from the intended image.
There is a need for an approach to preventing stray light ghost images arising from catadioptric lenses from interfering with the sensing of a scene in an optical system. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system having a dome and, depending upon the type of dome, an optical corrector. Light reaches a sensor by passing through an optical train that includes a solid catadioptric imaging lens (sometimes termed a solid xe2x80x9ccatadiopticxe2x80x9d or xe2x80x9ccatxe2x80x9d lens). The solid catadioptric imaging lens is modified so avoid ghost images that otherwise may reach the sensor and be misinterpreted. The functionality of the solid catadioptric imaging lens is not adversely affected. The present approach does not require any modification of the sensor and avoids the need to expend large amounts of computational power to distinguish ghost images.
In accordance with the invention, an optical system comprises an outer dome, and a detector system comprising an optical train including a solid catadioptric imaging lens which is symmetric about a lens axis. The solid catadioptric imaging lens has a bore therethrough coincident with the lens axis. (The catadioptric imaging lens is a xe2x80x9csolidxe2x80x9d catadioptric imaging lens, meaning that it incorporates reflective and refractive elements into a single physical component.) The detector system further includes a sensor disposed to receive an optical ray passing sequentially through the outer dome and the optical train.
The bore is positioned to block the stray light rays that originate due to the ghost effect in the optical system due to the presence of the solid catadioptric imaging lens. The bore maximum diameter is sufficiently small that the bore does not interfere with the light rays from the scene that are of interest. The stray light originating from this source is therefore removed from the optical path before it can reach the sensor.
The bore may be cylindrically symmetric or have another shape such as a frustum of a cone. The bore may be filled with an opaque material, or it may have its interior surface ground. The bore preferably has an interior surface whose maximum distance from the lens axis is substantially equal to an inner rim ray diameter of the optical system.
The outer dome may comprise a segment of a sphere, in which case no optical corrector is needed in most instances. The outer dome may not be a segment of a sphere, in which case an optical corrector may be positioned between the dome and the detector system. The optical corrector comprises a transparent body having an optical corrector shape responsive to a shape of the outer dome and positioned in an optical path between the outer dome and the detector system.
Preferably, the detector system is mounted on a movable optical train support. The movable optical train support is typically a gimbal, such as a rollnod gimbal or an x-y gimbal.
To achieve further reduction in stray light, the optical system may include at least one baffle positioned in the optical path between the outer dome and the detector system and fixed in space relative to the central axis, each baffle comprising a frustoconical tube that is rotationally symmetric about the central axis.
The present approach allows the use of the efficient solid catadioptric imaging lens in the optical train, but avoids the potential problem with ghost images. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.